1. Field of the Invention
The present invention relates to a technique for producing a high-resolution phase-shift masking photomask, and is specifically directed towards enabling the use of an electron beam exposure tool to perform the phase level patterning operation during the mask-making process.
2. Related Art
Photomasks are a key element in the manufacture of modern integrated circuits (ICs). During lithography process steps, photomasks are used to transfer an IC layout onto a wafer surface. Each photomask typically comprises a pattern etched into a pattern layer (typically chrome) on a quartz substrate, the pattern representing one layer of the IC. Accordingly, the accuracy of the projected images formed by the photomask strongly affects the performance of the final IC.
During a lithography process step, exposure radiation (light) is used to project the pattern in the chrome layer of a photomask onto a wafer. The chrome layer pattern comprises a multitude of tiny apertures in the chrome layer. As the dimensions of modern IC devices continue to shrink, the chrome apertures must become smaller and smaller, which leads to increasing diffraction of the exposure radiation as it passes through the photomask. This diffraction can cause projected images from adjacent apertures in the chrome layer to overlap and merge, preventing the desired pattern from properly resolving at the wafer surface. Various techniques have been developed in an effort to extend the usable range of optical lithography tools. One of the most important of those techniques is phase-shift masking (PSM) technology.
In a PSM photomask, critical features are defined using pairs of complementary features (apertures) in the chrome layer. The complementary features are configured such that the exposure radiation transmitted by one aperture is 180 degrees out of phase with the exposure radiation transmitted by the other aperture. Therefore, rather than constructively interfering and merging into a single image, the projected images destructively interfere where their edges overlap, creating a clear separation between the two images. As a result, the images formed by a PSM photomask can have a much higher resolution than images formed by a conventional photomask.
To shift the phase of the exposure radiation passing through a given aperture in the chrome layer, a pocket is etched in the quartz substrate at that aperture. The quartz pocket reduces the thickness of the quartz substrate at the out-of-phase (xe2x80x9cphasedxe2x80x9d) aperture relative to the quartz substrate thickness at the in-phase aperture. The pocket depth at the phased aperture depends on the wavelength of the exposure radiation used by the lithography tool in which the PSM photomask is to be used. By precisely configuring the relative thickness of the quartz substrate at the phased and in-phase regions, the projected images from adjacent apertures can be set to be 180 degrees out of phase.
FIGS. 1A-1F depict a conventional process for creating a PSM photomask. FIG. 1A shows a conventional photomask blank 110 comprising an original resist layer 113 formed over a chrome layer 112, which is in turn formed over a quartz substrate 111. During a primary patterning operation, an electron beam (xe2x80x9ce-beamxe2x80x9d) scanner exposes regions 113a and 113b of original resist layer 113. Exposed regions 113a and 113b are developed away, leaving patterned resist layer 113 shown in FIG. 1B. An etch process is then performed, thereby transferring the pattern in original resist layer 113 into chrome layer 112. Original resist layer 113 is then stripped away, leaving patterned chrome layer 112 with apertures 112a and 112b, as shown in FIG. 1C.
At this stage, the entire layout pattern is contained in chrome layer 112. However, to complete the PSM photomask, quartz substrate 111 must be etched to the proper depth under the out-of-phase, or xe2x80x9cphasedxe2x80x9d, portion of the layout pattern. As shown in FIG. 1D, a secondary resist layer 140 is formed over patterned chrome layer 112, and a portion 140a of secondary resist layer 140 is optically exposed during a xe2x80x9cphase level patterningxe2x80x9d operation. Exposed portion 140a is developed away, and chrome layer 112 is etched through, as shown in FIG. 1E. Thus, the actual etching of quartz substrate 111 is controlled by aperture 112b in chrome layer 112, i.e., the purpose of the phase level patterning operation is merely to uncover the appropriate apertures in chrome layer 112. After the quartz etch, secondary resist layer 140 is stripped, leaving a basic PSM photomask 110f as shown in FIG. 1F.
Once the quartz etch is complete, apertures 112a and 112b are complementary apertures, as the images projected by the two during a lithography process step will be 180 degrees out of phase with each other. Aperture 112a is designated the in-phase aperture, whereas aperture 112b is designated the phased aperture. The phase shift of aperture 112b is provided by a pocket 111b that thins quartz substrate 111 under aperture 112b. While only a single complementary pair of apertures is depicted, any number could be present in an actual PSM photomask, each of the phased apertures having a quartz pocket of depth d.
In this manner, basic PSM photomask 110f is configured to produce properly phase-adjusted images at the wafer surface. However, the pockets formed by the quartz etch affect not only the phase, but also the intensity, of the exposure radiation transmitted by the PSM photomask. FIG. 2 shows how PSM photomask 110f would be used in a lithography process step. PSM photomask 110f of is placed xe2x80x9cupside downxe2x80x9d (i.e. with quartz substrate 111 on top) in a stepper (not shown), and exposure radiation from the stepper projects the pattern in chrome layer 112 onto a wafer (also not shown). As indicated in FIG. 2, diffraction of the exposure radiation at phase layer aperture A1 starts to occur at the base of pocket 111b, whereas diffraction at chrome aperture A2 originates at the surface of quartz substrate 111 (i.e. where aperture A2 meets quartz substrate 111). Therefore, more of the exposure radiation is xe2x80x9clostxe2x80x9d inside quartz substrate 111 at phase layer aperture A1, resulting in unequally-sized and improperly-spaced projected features. This phenomenon is described in detail in xe2x80x9cPhase-Shifting Mask Topography Effects on Lithographic Image Qualityxe2x80x9d by Pierrat et al., IEDM 92-53, IEEE 1992, herein incorporated by reference.
To overcome this problem, a post-processing step is typically performed on a PSM photomask after the phase layer quartz etch. The purpose of the post-processing step is to create an xe2x80x9cundercutxe2x80x9d beneath the chrome layer by increasing the width of the quartz pockets under the phased apertures. This undercutting process is typically accomplished by performing a wet (isotropic) etch on the quartz layer. For example, after the dry (anisotropic) etch shown in FIG. 1E, quartz substrate 111 could be wet etched, as shown in FIG. 1G (the dry etch depth would have to be reduced to maintain the final phase-shifting characteristics of the quartz after the wet etch). The wet etch gives the pocket in quartz substrate 111 a final width W1, which is greater than the width W2 of the aperture in chrome layer 112, as shown in FIG. 1H. The diffraction effects from the base of the pocket in the quartz substrate then have much less effect on the actual radiation transmitted by the aperture in the quartz layer. This in turn allows the intensity characteristics of the radiation output from the phase layer aperture to more closely match the characteristics of the output from the chrome aperture, thereby resulting in improved PSM functionality.
This post-processing step can be performed in various ways. For example, after a dry etch of the quartz substrate to form the phase layer (as shown in FIGS. 1E-1F), a wet etch could be performed on both the phase and pattern layers, as shown in FIG. 1I. This would produce a chrome layer overlap at all apertures in chrome layer 112, as shown in FIG. 1J, enhancing the similarity of the transmitted radiation intensities from those apertures. Alternatively, after a dry etch of the quartz substrate to form the phase layer (as shown in FIGS. 1E-1F), a second dry etch could be performed on both the phase and pattern layers, as shown in FIG. 1K, to extend the quartz pockets beneath all apertures in chrome layer 112, as shown in FIG. 1L. A wet etch could then be performed on all those quartz pockets to create the desired chrome layer overlap, as shown in FIG. 1M. Because the absolute difference between quartz substrate thicknesses at both the phase and pattern layers remains unchanged, the relative phase of transmitted light from both regions remains the same. At the same time, because the relative difference between the phase layer and pattern layer quartz cavity depths is reduced (i.e. the ratio between the depths moves closer to one) the similarity of the transmitted radiation intensity is enhanced.
As noted previously in describing FIG. 1D, optical exposure tools are typically used to expose the portions of the PSM photomask to be etched to form the phase layer. Optical exposure tools are generally less precise than e-beam exposure tools (i.e., have a lower resolution and produce greater corner rounding in exposed images), but have been acceptable for this secondary, or xe2x80x9cphase levelxe2x80x9d patterning operation because the actual phase level etch is governed by the existing apertures in the chrome layer. The patterning operation must merely be precise enough to uncover the relevant chrome apertures. However, as modern device geometries continue to shrink, optical tools can no longer accurately uncover the smaller chrome apertures. For example, the round-cornered features produced by optical exposure tools during phase level patterning operation may xe2x80x9ctrimxe2x80x9d the sharp corners of the chrome layer features formed by an e-beam exposure tool. The subsequent quartz etch to form the phase layer would not etch those corners, adversely affecting the final performance of the PSM photomask. Therefore, it is desirable to be able to use e-beam exposure tools for the phase level patterning operation.
Unfortunately, conventional PSM photomask production techniques cannot readily incorporate an e-beam process for phase level patterning. As the e-beam tool scans across the portions of the resist to be exposed, electrons penetrate into the resist layer. If allowed to accumulate, these embedded electrons can deflect the path of the e-beam, thereby resulting in targeting errors. This xe2x80x9ccharging problemxe2x80x9d does not arise during the primary patterning operation (i.e., the resist patterning prior to chrome etch), because of the continuous chrome layer under the original resist layer. Specifically, because the chrome layer typically has an external connection to ground during the patterning operation, the chrome layer provides an electrical conduction path that serves to dissipate any charge buildup in the resist layer.
However, during the phase level patterning operation, the chrome layer is no longer continuous. At each aperture in the chrome layer, the secondary resist layer is formed directly on the non-conducting quartz substrate. Consequently, charge buildup in the secondary resist layer at the chrome aperture locations has no dissipation path. This charging issue is especially problematic because the chrome aperture locations are precisely the regions that need to be exposed during the phase level patterning operation. This problem is illustrated in FIG. 3, which depicts the phase level patterning operation shown in FIG. 1D, but with an e-beam exposure process rather than an optical exposure process. As shown in FIG. 3, as the e-beam scans across a target region 140b of resist layer 140, charges (i.e. electrons) build up in the resist. Because target region 140b overlies chrome aperture 112b, and is actually contacting non-conducting quartz substrate 111, there is no dissipation path for the electrons in target region 140b. Therefore, a local charge develops that can eventually deflect the incoming e-beam off its intended path, and thus lead to inaccurate patterning of resist layer 140.
Conventional attempts to overcome this charging issue incorporate the temporary addition of a conductive layer over the secondary resist layer. FIGS. 4A-4D show a method that can be used to enable the use of an e-beam tool to perform the phase level patterning operation. FIG. 4A shows a patterned chrome layer 412 on a quartz substrate 411. Chrome layer 412 includes apertures 412a and 412b, formed in previous process steps substantially similar to those described in relation to FIGS. 1A-1C. A secondary resist layer 440 is formed over chrome layer 412, and a temporary conductive layer 450, such as a metal layer or conductive polymer layer, is deposited over secondary resist layer 440. Conductive layer 450 can be penetrated by the e-beam, and so does not interfere with the exposure of secondary resist layer 440 during the patterning operation. At the same time, conductive layer 450 provides a charge dissipation path for any electron buildup in secondary resist layer 440, thereby preventing localized effects on e-beam targeting. Therefore, the e-beam tool can accurately expose a desired portion 440a of secondary resist layer 440.
After the exposure step, conductive layer 450 must be removed using an appropriate process (e.g., an etch process for a metal layer, a solvent strip process for a conductive polymer), as shown in FIG. 4B. Once conductive layer 450 is removed, exposed portion 440a of secondary resist layer 440 can be developed away, uncovering phased aperture 412b but leaving in-phase aperture 412a covered, as shown in FIG. 4C. A quartz etch process can then be performed to create a quartz pocket 411b under aperture 412b, after which secondary resist layer can be stripped, leaving in-phase aperture 412a and phased aperture 412b properly configured, as shown in FIG. 4D. Of course, a post-processing wet etch step could then be performed to improve PSM performance.
While the aforementioned process does allow the more accurate e-beam exposure tool to be used for the phase level patterning operation, additional conductive layer formation and removal steps are required. This undesirably adds both cost and complexity to the mask making process. In addition, the process can be further complicated if a barrier layer is required between secondary resist layer 440 and temporary conductive layer 450 to prevent interactions between the two layers during formation or removal of conductive layer 450. Accordingly, it is desirable to provide a method for efficiently producing a PSM photomask that permits e-beam tool usage during the phase level patterning operation without introducing additional process steps.
The present invention provides a method for producing a PSM photomask that allows the use of an e-beam exposure tool during the phase level (secondary) patterning process, thereby enabling higher precision patterning than is available with optical exposure tools. The invention avoids the secondary resist layer charging problem typically associated with e-beam exposure tools by providing a conductive layer on the quartz surface that dissipates any charge buildup during phase level patterning.
A method for producing a PSM photomask in accordance with an embodiment of the invention incorporates a photomask blank that includes an extra conductive layer between the chrome (pattern) layer and the quartz substrate. Such blanks are sometimes used as calibration blanks to measure pattern placement errors in e-beam exposure tools, but are not used as photomask blanks (i.e., are not used to produce actual photomasks). The added conductive layer is typically opaque, and must be removed at the apertures patterned into the chrome layerxe2x80x94an undesirable extra step in conventional mask making processes.
According to an embodiment of the invention, the resist layer of a conductive mask blank is patterned using an e-beam exposure tool with the in-phase and phased features of an IC layout. The pattern is then etched into the chrome layer, leaving the conductive layer intact. A second resist layer is formed over the patterned chrome layer, and is patterned to uncover the phased features in the chrome layer. This phase level (secondary) patterning operation is performed using an e-beam exposure tool, since the continuous conductive layer provides a charge drainage path for any electron buildup in the second resist layer. As noted previously, an e-beam tool advantageously provides more accurate patterning than an optical exposure tool.
After the phase level patterning operation, an anisotropic etch etches through the conductive layer and creates pockets in the quartz substrate under each phased feature in the chrome layer. The depth of this xe2x80x9cphase level etchxe2x80x9d is dependent on the wavelength of the exposure radiation, the index of refraction of the quartz substrate, and the relative etch rates of the conductive layer and quartz substrate. By etching to the proper depth at this stage, proper PSM functionality will be provided by the final photomask.
After the anisotropic etch, the second resist layer is stripped, and an isotropic etch is performed through both the in-phase and phased features in the chrome layer to complete the PSM photomask. As noted previously, the purpose of this anisotropic etch step is to undercut the features in the chrome layer to reduce diffraction-induced intensity variations in the images projected by the final mask. However, this xe2x80x9cpost-processingxe2x80x9d step also eliminates any problems associated with the opacity of the conductive layer. Because the isotropic etch removes the conductive layer at the in-phase features in the chrome layer, and because the previous anisotropic etch removed the conductive layer at the phased features, the conductive layer is completely removed from the path of the exposure radiation through the photomask.
According to another embodiment of the invention, an anisotropic etch can be performed through both in-phase and phased features in the chrome layer after the phase level etch. A subsequent isotropic etch provides the desired undercutting at the features in the chrome layer. The additional anisotropic etch results in reduced quartz substrate thickness at both the in-phase and phased chrome features, thereby enhancing equalization of transmitted radiation intensity.
According to another embodiment of the invention, a blank having a xe2x80x9cvisually transparentxe2x80x9d conductive layer can be used to create a PSM photomask. Indium oxide doped with tin oxide, or xe2x80x9cITOxe2x80x9d, is transparent at visible wavelengths (i.e., visually transparent), but is opaque to the short wavelength radiation used by mask patterning tools. Display manufacturers often use a layer of ITO in their flat-panel displays to provide electrical routing capability without interfering with the imaging properties of the display. A PSM photomask blank in accordance with an embodiment of the invention could therefore be made with a layer of ITO between the chrome layer and the quartz substrate. By making use of such a blank, the phase level etch can be an isotropic etch, since the conductive layer at the in-phase features in the chrome layer allows good transmission of the exposure radiation used in lithography process steps. Note that the depth of the phase level etch under the phased chrome features must account for any difference in the indices of refraction of the transparent conductive layer material and the quartz substrate.
In this manner, the charging issue associated with conventional PSM photomask-making operations can be overcome. The resultant improved patterning precision is achieved without requiring additional conductive layer formation and removal operations, greatly simplifying the mask making process. Consequently, there is much less chance for defect formation, providing a much more robust process. The present invention will be more fully understood in view of the following description and drawings.